<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        * {
            margin: 0px;
            padding: 0px;
        }
        
        body {
            background-image: url(images/bg1.jpg);
        }
        
        #mask {
            background-color: rgba(255, 255, 255, 0.3);
            height: 200px;
            text-align: center;
        }
        
        #mask img {
            width: 200px;
            margin-top: 35px;
            cursor: pointer;
        }
    </style>
</head>

<body>
    <div id="mask">
        <img src="images/bg1.jpg" alt="">
        <img src="images/bg2.jpg" alt="">
        <img src="images/bg3.jpg" alt="">
    </div>
    <script src="common.js"></script>

    <script>
    var mask = my$("mask");

    var imgs = mask.children;

    for (var i = 0; i < imgs.length; i++) {
        var  img = imgs[i];

        img.onclick = clickHandle;
    }


    function clickHandle() {
        var src = this.src;
        document.body.style.backgroundImage = "url(" + src + ")";
    }

</script>

</body>

</html>